Interactions between humic acid (HA), extracted from Brazilian peat soil, and cations of copper(II) and zinc(II) have been examined by infrared spectroscopy, and electrochemical and thermogravimetric analyses. Spectral studies revealed that the interaction with metals occurred mainly at the carboxylic acid groups of HA. The stability constants of HA complexes formed with Cu(2+) and Zn(2+) were found, by square wave voltammetry and application of the Lingane model, to be 8.93 x 10(10) and 2.92 x 10(2), respectively. Thermal analysis indicated that the stability of HA was increased by the presence of divalent cations of copper and zinc.